1. Field of the Invention
The present invention relates generally to recording and reproducing information using a magnetic medium, and more particularly, to a magnetic recording medium constructed so that its various layers have particular constructions; a method for producing such a magnetic recording medium; and a recording system incorporating such a magnetic recording medium.
2. Description of the Related Art
Accompanying the trend towards smaller sizes and higher operating speeds in modern electronic computers, the increased capacity and access speed of magnetic disk devices and other external storage units has been in strong demand. In particular, high-density and high-speed magnetic disk recording devices have been more strongly demanded than ever before.
Known magnetic recording media useful in magnetic disk devices can be divided into recording media of the coated type, obtained by coating a disk substrate with a powder of a magnetic material oxide, and recording media of the thin-film type obtained by depositing a thin film of a magnetic metal onto the substrate. The recording media of the thin-film type generally have a higher density of magnetic material in the recording layer than those of the coated-type, and are hence more suited for high-density recording.
A Co-based alloy having a large magneto-crystalline anisotropy has chiefly been used as the magnetic material for the thin-film type recording medium. The Co-based alloy has a crystal structure of the hexagonal close-packed type (hereinafter HCP), has a direction of easy magnetization which is in parallel with the c axis of the HCP structure, and has a large coercivity H.sub.c, a remanence (residual magnetic flux density) B.sub.r, and a squareness S, S* in the same direction.
Currently, the direction of easy magnetization is substantially parallel with the surface of the disk. In forming the medium, therefore, the magnetic layer crystals must be so grown that the c axis of the Co-based alloy is nearly parallel with the disk surface.
Conventionally, a Cr layer or a Cr-alloy layer has been employed as an underlayer of the magnetic layer. Such an underlayer conventionally has a body-centered cubic structure (hereinafter BCC). The crystal orientation of the underlayer greatly affects the crystal orientation of the magnetic layer.
For instance, Japanese Patent Laid-Open No. 1-220217 discloses a magnetic recording medium in which the (110) crystal lattice plane of the Co-based alloy and the c axis are formed in parallel with the surface of the substrate on a Cr underlayer of 5 nm to 20 nm thickness, and of which the (100) crystal lattice plane is in parallel with the surface of the substrate.
Japanese Patent Laid-Open No. 63-197018 discloses that, when Ti or Si is added to the Cr underlayer, the crystal orientation of Cr becomes isotropic on the surface of the disk, and modulation of output waveform decreases. Furthermore, Japanese Patent Laid-Open No. 62-257618 discloses that the addition of V or Fe to Cr increases the lattice constant, and matching is improved between the (100) plane of Cr and the (110) plane of the Co-based alloy of the magnetic layer, enabling the coercivity and squareness to be increased.
Conventionally, a variety of underlayers have been employed to improve magnetostatic parameters such as the coercivity and squareness, without, however, paying much attention to such read/write dynamic magnetic parameters as noise, read output and resolution. Moreover, the conventional thin-film type recording media are incapable of producing a sufficiently large resolution and signal-to-noise (S/N) ratio for writing and reading, particularly for data read and written using a magnetoresistive (MR) head featuring high read sensitivity, as in the known inductive write/MR read thin-film magnetic head.
Therefore, the art strongly demands a medium having excellent dynamic properties in order to further improve performance of the magnetic disk device.
Further, conventional magnetic recording media have employed aluminum alloy, glass, organic resin, ceramic or similar materials as the disk substrate for the thin-film type recording media. To improve the surface hardness of the substrate and its magnetic characteristics, an anodic oxide film, or an Ni--P layer, have been formed on the disk substrate surface to a thickness of about 15 .mu.m by a plating method.
On the surface of such a plated substrate, fine grooves have been formed substantially in the circumferential direction, as described in, for example, Hedgcoth, U.S. Pat. No. 4,735,840, and in Japanese Patent Laid-Open Nos. 61-29418, 62-146434 and 63-121123. Such fine grooves are usually called "texture" and are formed by polishing the surface of the disk in nearly the circumferential direction by using abrasive grains having grain diameters of 1 to several microns.
Texture decreases the effective contact area between the magnetic head and the medium when revolution is stopped, and thus decreases the coefficient of friction between the head and the medium. Depending upon the shape and the size of the texture, furthermore, sticking of the head to the recording medium at the start of disk rotation is suppressed.
Additionally, the deposition conditions, such as heating temperature and method of transport within the deposition chamber, can cause nonuniformity of the magnetic anisotropy in the generally circumferential direction, and a consequent change in read output. By optimizing the texture size, underlayer composition and magnetic layer composition, as well as the deposition conditions, the magnetic anisotropy can be made uniform in the generally circumferential direction by applying texture. Moreover, magnetic fluctuation, or modulation, in the read/write output can be suppressed.
Thus, the aforementioned conventional texture is effective for improving wear resistance and read/write characteristics of the thin-film type recording media. However, its beneficial effects greatly vary depending upon the microscopic contours of the texture surface. To form a medium having uniform magnetic characteristics, therefore, the microscopic shape of the texture must be made uniform. However, as mentioned, the conventional texture has been formed by using abrasive grains having grain diameters of 1 to several microns, and thus it has been difficult to uniformly control fine surface ruggedness or pitch, maintaining a size of smaller than 1 .mu.m. Therefore, the coefficient of friction between the head and the medium at the start of disk rotation, and the read/write characteristics, undergo a change from disk to disk and from lot to lot.
Additionally, to carry out even higher density reading and writing, the recording track width must be decreased below that of the currently-employed track width. In this case, however, nonuniformity in the shape of the texture in the radial direction of the disk can further seriously affect read and write signals. In particular, when uniformity of read/write characteristics in the radial direction are strictly required, such as in the case of positioning servo signals, conventional texture cannot be employed.
The gap between the magnetic head and the recording medium and the magnetic field profile of the medium are two additional parameters that must be optimized to increase recording density. However, conventional texture has resulted in increased contact between the magnetic head and the recording medium, due to fine protrusions that are unavoidably formed on the disk substrate surface by the texturing. One prior art approach to solving this problem has been to polish the protrusions from the substrate surface (see Japanese Patent Laid-Open No. 1-162229). Alternatively, the conditions for forming the texture can be controlled so that the ruggedness in the texture is small. In this case, however, the contact area increases between the magnetic head and the medium, leading to the inevitable sticking of the head during the beginning of disk rotation.
Japanese Patent Laid-Open No. 1-273218 teaches a method of decreasing the dynamic coefficient of friction by forming intersecting textures in the circumferential direction. However, the magnetic characteristics of the resulting magnetic layer are not sufficiently improved compared with those of the media having the customary texture, and modulation is likewise not sufficiently suppressed.
Further, to improve the signal-to-noise (S/N) ratio for writing and reading at high linear recording density, the magnetic layer of the medium may be formed as separate thinner magnetic layers using non-magnetic intermediate layers formed between each thinner magnetic layer. Japanese Patent Laid-Open Nos. 63-146219, 1-173313, 1-217723, and 2-281414 teach such multilayered media, which show a better performance in read and write. The media noise is remarkably reduced because of the reduction of each magnetic layer thickness, giving an increase in the S/N ratio. However, the read output at a high linear recording density tends to decrease, resulting in a decrease in the resolution. Thus, the conventional multilayered media still leave a strong demand for increased S/N ratio at a high linear recording density.